Filter clog sensing system and method for compensating in response to blower speed changes

ABSTRACT

A sensing system and method for use with a gas filter in a gas conduit attached to a gas blower, having a motor for producing a first gas flow blower speed, discards readings taken when the blower is off or during blower startup and shutdown, and recalculates a clog threshold in response to changes in the fan speed of a multi-speed blower.

BACKGROUND

Embodiments described and claimed herein relate to monitoring to detect clogging in a gas filter, which is utilized in a gas conduit attached to a gas blower, the latter having a motor for producing a first gas flow. Example applications include filter monitoring for Heating, Ventilation, and Air Conditioning (HVAC) systems. Such systems are used for a variety of functions, from simple ventilation to heating, cooling, and humidity control.

Generally, the purpose of a HVAC system is to move air for temperature and environmental comfort within a house, building, or factory (collectively referred to herein as building). The same purpose applies to automobiles and other structures in which healthy, comfortable air/gas quality is necessary or desired. In a HVAC system, a blower, having a motor for producing a first gas flow, moves air in the gaseous phase through one or more gas conduits (ducts) to different locations within a building before it exits the duct through a vent. After entering a duct through a HVAC system's air intake, air flows in response to a blower. Typical blowers have a motor that rotates a fan having a plurality of blades, in order to pull air through a duct in a direction that can be thought of as moving from upstream to downstream.

Many HVAC systems use filters to remove dust, dirt, contaminants, and other undesired particles that adversely affect air quality so they are not delivered past a certain location within the duct, and do not exit through a vent. Filters limit the progress of undesired particles through a duct, for example by physical restriction in which the small size of openings in a filter keeps particles from progressing through and past it, or by electrostatic attraction that hold particles to a filter.

As a filter collects undesired particles, it becomes discolored and dirty. Although a dirty filter does not substantially affect flow of air through a duct, over time, the accumulation of undesired particles produces clogging in a filter, which substantially decreases air flow in the building, and causes dust and dirt to accumulate inside the duct system and on the blower fan blades, all of which potentially reduces the quality of air in a building and adversely affects the performance of the HVAC system. Consequently, filters in systems such as HVAC systems, as well as automobiles, must be changed periodically.

Visual inspection is one way to determine when a filter should be changed, but it is time- and labor-intensive. Therefore, it is desirable in many situations to automatically sense whether a filter should be changed. This is often done by evaluating the condition of a filter as a function of measurable conditions that are related to air flowing through a duct. For example, one such air treatment system is described in U.S. Pat. No. 7,178,410, titled Clogging Detector for Air Filter, the entire disclosure of which is incorporated herein by reference. This patent discloses a filter clog detector, in which a first temperature sensor, which is coupled to a heater, and a second temperature sensor, which is unheated, are positioned in a duct. The heated sensor is kept at a temperature higher than the unheated sensor by a fixed number of degrees. If air flow velocity is greater, then more energy is dissipated away from the heated sensor. Accordingly, a measurement of the voltage required to maintain the temperature difference between the two sensors indicates the amount of air flowing through the duct where the sensors are located. Using techniques that are known to persons of ordinary skill in the art, including but not limited to those disclosed in the above-referenced patent, filter condition can be determined based on voltage readings, or similar properties having a relationship to air flow through a duct.

Other examples, regarding how filter condition is determined as a function of measurable properties or conditions within a duct, include differential pressure sensors, which respond to a pressure drop that occurs from the upstream side of a filter compared to the downstream side. Even if only to a slight degree, a new, or clean, filter restricts air flow through a duct. This restriction produces a corresponding pressure on the upstream side of the filter. Further, air passing through the filter also produces pressure on the downstream side. These pressures are measured using sensors and methods that are known to those having ordinary skill in the art. For a clean conventional filter, the differential pressure from the upstream side of the filter to the downstream side will usually be relatively small. However, as a filter becomes dirty and then clogged, the restriction on air that passes through the filter tends to decrease pressure on the upstream side. Further, because less air passes through a clogged filter, this tends to correspond to an increase in pressure on the downstream side.

Although various techniques have been disclosed for determining filter condition, as a function of measurable conditions within or related to a duct, other variables besides the extent of filter clogging may affect air flow in a duct, as indicated by measurable properties such as the voltage sensor as discussed above. For example, changes in blower speed may either increase or decrease the amount of air pulled into and through a duct. Multi-speed blowers differ from single-speed blowers and two-speed blowers, the latter of which use a two-speed motor, e.g., one speed for summer cooling and another speed for winter heating. Multi-speed blowers are capable of operating at any of a number of different speed settings. Many blowers require a period of time to stabilize and reach a consistent operating speed. For example, some blowers require approximately 60 seconds for this to occur.

Consequently, when determining when a filter should be changed, it is desirable to ascertain that readings of measurable conditions in a duct are actually indicative of a change in filter condition over time. In other words, it is desirable to prevent there from being an indication of clogging merely because of changes in blower speed.

SUMMARY OF INVENTION

Multiple embodiments and alternatives disclosed herein relate to a sensing system for monitoring a gas filter's condition. Applications include various kinds of air handling systems. One purpose of the multiple embodiments and alternatives is for determining when a gas filter should be changed based upon its level of clogging, where such filter is used with a HVAC system or other type of gas conduit attached to a gas blower, having a motor for producing a first gas flow. The embodiments and alternatives disclosed herein compensate for and generally help to avoid the potentially compromising effects on accuracy and efficiency due to changes in blower speed, which have the potential to either mimic a clogged filter, or hide the effects of a clogged filter.

In general, these purposes are accomplished by comparing measurements at a prior time of calibration to measurements at a later time of monitoring, so that usable readings are kept and others are discarded. Throughout the descriptions and teachings herein, a reading pertains to a property that is measured. In this sense, the words measurement (or measure) and reading (or read) are used interchangeably herein. Measurements taken during monitoring events are compared to earlier measurements to determine if a blower's speed has changed. If so, the sensing system recalculates a clog threshold in response to the change. Embodiments and alternatives as disclosed herein do not require a second sensor for sensing air flow or blower fan speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a portion of a conventional HVAC system in which clogging of a gas filter is monitored, according to multiple embodiments and alternatives.

FIG. 2 illustrates a portion of a conventional HVAC system, in which clogging of a gas filter is monitored, according to multiple embodiments and alternatives.

FIG. 3 is a cross-sectional view of a sensing tube as illustrated in FIG. 1, according to multiple embodiments and alternatives.

FIG. 4 is a circuit diagram depicting a sensing tube of the type illustrated in FIG. 3, according to multiple embodiments and alternatives.

FIG. 5A is a flow chart related to calibrating a sensing system for use in monitoring a gas filter, according to multiple embodiments and alternatives.

FIG. 5B-5C is a flow chart concerning monitoring of a gas filter using a sensing system, according to multiple embodiments and alternatives

FIG. 6 is a graph that plots sensor voltage level against the level of cleanliness of a gas filter in a HVAC system, according to multiple embodiments and alternatives.

MULTIPLE EMBODIMENTS AND ALTERNATIVES

FIG. 1 is a simple diagram showing a portion of a gas conduit 10 having blower 11 that pulls air in a direction indicated by arrow 12. Gas filter 14 is positioned within duct 10. Sensing system 15 generally consists of sensing tube 16 and support base 17. In some embodiments, sensing tube 16 has an inlet 25 that ambient air from outside duct 10 flows into, and an outlet 27 providing fluid communication with the interior of duct 10. Generally, therefore, sensing tube 16 may be positioned at least partially within, or flush with, duct 10. In some embodiments, it comprises a cylindrical tube with openings at both ends (inlet 25 and outlet 27, respectively), which enable blower 11 to pull ambient air into duct 10.

In FIG. 1, sensing system 15 is incorporated with a duct at a position downstream relative to filter 14. Consequently, air entering duct 10 passes through filter 14 before it flows past the position of sensing tube 16 in the interior of duct 10. Alternatively, in some embodiments, sensing system 15 is incorporated with the duct at a position upstream relative to filter 14, meaning that air entering duct 10 will flow past sensing tube 16 before progressing through and past filter 14.

In some embodiments, sensing system 15 is attached to duct 10 by forming a hole in the wall of the duct appropriately sized to accommodate sensing tube 16. Support base 17 is also mounted to the external wall with mounting screws so as to maintain operable contact with sensing tube 16, including its various hardware and software components as described herein.

FIG. 2 illustrates an alternative means for sensing the condition of a filter, which involves taking a pressure differential across filter 14. Accordingly, differential pressure sensor 20 comprises a differential pressure sensor support base 19, first tube 21 positioned upstream relative to filter 14, and second tube 23 positioned downstream. In some embodiments, tube 21 has the same components arranged as those of sensing tube 16. Tube 21 and tube 23 thus are arranged to measure differential pressure across filter 14, using techniques as are known to persons of ordinary skill in the art.

As illustrated in both FIG. 1 and FIG. 2, embodiments disclosed herein also include a processor 29, which acts as a controller, and which can be physically integrated with support base 17 or differential pressure sensor support base 19, respectively. In some embodiments, processor 29 includes one or more general or special purpose microprocessors, or any one or more processors of any kind of digital computer, including ones that sense conditions, and perform various calculations as further discussed herein. Processor 29 further includes, or is communicatively coupled to, computer readable storage medium such as, for example memory, which may optionally include read-only memory (ROM), random access memory (RAM), non-volatile RAM (NVRAM), optical media, magnetic media, semiconductor memory devices, flash memory devices, mass data storage device (e.g., a hard drive, CD-ROM and/or DVD units) and/or other storage as is known in the art. Additionally, processor 29 includes, or is communicatively coupled to, memory having computer executable program instructions which, when executed by processor 29, cause it to perform the steps as described herein. In some embodiments, processor 29 is coupled to complementary components, for example user interface screens, key pads, light indicators, and/or dip switches (these are not shown) responsive to operator input to allow user control of the sensing system. Preferably, components such as screens, key pads, and light indicators are integral with support base 17 or 19, respectively. Some embodiments utilize a dip switch configured for setting the system to upstream mode or downstream mode, and / or switching between such modes depending on where a person chooses to install sensing system 15 in relation to filter 14 (see FIG. 1).

As FIG. 3 illustrates, in some embodiments, sensing tube 16 further includes an outer plate 24, preferably formed of an insulator, for separating an interior space 22 of sensing tube 16 from the interior of duct 10. In some embodiments, heater 28 is electrically coupled to heated sensor 30, both of which, along with unheated sensor 32, are positioned in the interior 22. Sensors 30, 32 and heater 28 are connected via wires 33 to an electronic circuit, which can be external to interior 22. Alternatively, wireless connection is provided as known to persons of ordinary skill in the art. In some embodiments, plate 24 further comprises a thermal insulating material 26.

As noted, in some embodiments, sensing tube 16 is positioned partially within duct 10, such that one of its openings serves as an inlet 25 for ambient air, while the other serves as an outlet 27 in fluid communication with duct 10. Air flowing through sensing tube 16 dissipates energy from the space around both of sensors 30, 32, thus lowering the temperature at each sensor location. In some embodiments, sensing tube 16 is comprised of other features, for example interior 22 may be potted with thermally conductive epoxy (not shown), and heater 28, heated sensor 30, and unheated sensor 32 are affixed to an interior surface of plate 24 by techniques and methods known to persons of ordinary skill in the art, for example by soldering or with a suitable adhesive such as epoxy.

In some embodiments, heated sensor 30 is a thermistor, which is thermally sensitive to changes in temperature of ambient air flowing through interior 22. It will be noted that maintaining a temperature difference of no more than 5° C. provides significant advantages to the present embodiments, because less power is consumed to maintain such a difference than a system that requires, for example, a 50° C. temperature difference between the two sensors.

Generally, in operation, unheated sensor 32 remains at the temperature of ambient air flowing into and through sensing tube 16. However, heater 28 increases and maintains the temperature of heated sensor 30 to a predetermined level, e.g., 3° C.-5° C. above the temperature of ambient air flowing through sensing tube 16. If sensing system 15 is positioned downstream from filter 14, as shown in FIG. 1, then the effect of clogging in filter 14 generally pulls a greater volume of ambient air through sensing tube 16. This is because clogging of filter 14 tends to restrict air flow through the filter, resulting in a lesser volume of air downstream of filter 14 being pulled by blower 11. Consequently, a higher volume of ambient air is pulled through sensing tube 16, with corresponding increase in air velocity. Therefore, heater 28 supplies more voltage to maintain the fixed temperature difference, as the movement of greater volumes of air through sensing tube 16 dissipates more energy. Thus, the voltage required varies with, and is an indicator of, the clogging in filter 14.

Conversely, if sensing system 15 is positioned upstream from filter 14, these relationships are essentially reversed. In other words, with filter 14 being positioned between blower 11 and sensing tube 16, the extent of filter clogging tends to limit the amount of ambient air that contacts sensors 30, 32. As a result, less energy is dissipated, and voltage needed to maintain a fixed temperature difference is reduced.

FIG. 4 is a circuit diagram depicting a sensor of the type illustrated in FIG. 3, in connection with a clogging detector for a gas filter. Heated sensor 30, and unheated sensor 32, are represented by S2 and S1, respectively. Voltage required for maintaining a predetermined temperature difference between sensors 30, 32, as discussed above, is transmitted as sensor output of flex circuit 35 (which in some embodiments comprises a thermal insulator 26). In some embodiments, flex circuit 35 is electrically coupled to sensing system controller input, including a signal amplifier of the type as is known to persons of ordinary skill in the art. Because flex circuit 35 transmits a signal representing voltage for maintaining the temperature difference between sensors 30, 32, this value is indicative of the extent of clogging of filter 14. Alternatively, in systems that utilize differential pressure sensors, this output is indicative of pressure drop across filter 14, as a function of clogging.

As previously noted, filter clog detection involves the basic understanding that clogging limits or restricts the volume of air passing through a filter. However, in many HVAC systems, blower speed is another variable that may affect the volume of air passing through a filter. If all other factors are equal, then a higher blower speed produces more air flow through a duct generally, and through air filters positioned in the duct in particular. Consequently, there is a potential that changes in blower speed will mimic the effects of a clogged filter, or will hide the effects of a clogged filter. This could occur in several ways. One is when the blower is turned off. Having no air flowing through a filter, or anywhere else in the duct, would tend to mimic a filter that is substantially or completely clogged. Also, taking measurements when a blower is starting up, but has not yet stabilized to operating speed, may also mimic a clogged filter, or hide the effects of a clogged filter. Likewise, the same may occur if readings are taken when a blower is in the process of shutting down, and thus is operating at less than normal operating speed.

Some multi-speed blowers automatically change speed in response to demand, for example to 50%, 80%, or 100% of the motor's capacity, in order to promote more efficient operation. A blower motor may operate at less than 100% on a mild day because it promotes efficiency to cool the building gradually. Consequently, even when there has been no appreciable change in filter condition over time, readings taken at an operating speed of 50% will differ from ones at 80%, or 100%, and such differences can potentially mimic filter clogging that does not exist, or hide filter clogging that actually does exist. However, while applicable to multi-speed blowers capable of operating at two or more speeds, these are not the same as variable-speed blowers which, in some cases, are capable of having a rotational speed of the blower motor that is continuously and infinitely controlled at very small interval adjustments of, for example, a variance of 1-2% in blower speed.

In a HVAC system that utilizes a multi-speed blower, or two-speed blower, it is desirable to distinguish between readings that are useful for determining filter condition, versus ones that need to be discarded because of a tendency to mimic filter clogging, or hide the effects of clogging. FIG. 5 provides a flow chart showing various steps for promoting detection accuracy, according to multiple embodiments and alternatives. The steps relate to validating a measurement obtained during a monitoring event, either by determining that the gas blower was on, or verifying that the blower was at a stable operating speed, when a particular measurement was obtained, and/or verifying that the blower was operating at the same speed when a particular measurement was obtained relative to the blower speed at the time of a previous measurement. Such validation helps to prevent changes in blower speed from adversely affecting filter clog detection. Discussion is provided regarding calibration (FIG. 5A) and monitoring (5B to 5C) of filter 14, in a HVAC system in which sensing system 15 is positioned downstream of the filter.

In some embodiments, calibration occurs either when a new filter is installed, or when an existing filter is cleaned. Steps 100 to 130 are for initiating the calibration process and for confirming that sensing system 15 is powered-up in order to obtain readings. The teachings and principles described herein regarding FIGS. 5A-5C are directed to sensing system 15, but can be adapted to sensor 20 in differential pressure systems, as well. Optionally, at step 120, a user obtains a value for the clean filter corresponding to zero air flow and/or to air flow when the blower is operating. For the latter, a user turns the blower fan on, for example by increasing or decreasing temperature setting at a thermostat (not shown). Preferably, a user then waits a sufficient period of time, for example 60 seconds, to ensure that the blower has stabilized to operating speed.

In some embodiments, the steps for obtaining a value for zero air flow involve taking a plurality of readings and calculating an average of the readings, as shown at steps 140, 150. Generally, a plurality of readings are obtained over a relatively short period of time, for example ten seconds, and then averaged. Accordingly, the phrase, obtaining a value, as used herein means calculating the average value for all readings in a sample during a particular monitoring event. As discussed below, an average of the values obtained from several monitoring events is also used in some embodiments

Steps 160 to 210 depict obtaining readings for a clean filter with the blower operating. Upon completing calibration at step 210, values obtained during calibration are stored in memory, as are subsequently obtained values during monitoring, as explained in more detail below. Once calibration is completed, sensing system 15 transitions to standby mode (220) until the next scheduled monitoring event occurs. In this regard, a schedule of monitoring every twenty-four hours conserves energy and promotes efficiency compared to a schedule of every hour.

Once calibration is completed, periodic monitoring occurs beginning at step 250, according to a schedule determined by a manufacturer or, in some embodiments, as selectably desired by a user. The following example is with reference to sensing system 15, in which sensor voltage level (needed to maintain the predetermined temperature difference between sensors 30 and 32, respectively) is compared to a clog threshold value, which is stored in memory. Before doing so, however, various optional checks and tests are provided at steps 255 to 280, for example checking the battery (255) and providing an indication if the battery is low (258).

Similarly, proper sensor operation is checked (260) after applying power to sensing system 15 and enabling sensor voltage level detection. If voltage is detected at step 260, it indicates that the sensing system has passed this check (270). The system will then proceed to step 290 to obtain readings. If voltage is not detected, then a failure flag is set at step 275. Optionally, the system requires only one such failure before transitioning to alarm mode (280). In this way, indicator means include, but are not limited to, various kinds of alarms in the form of audible, visual, text, LED, and light cues. Alternatively, the system is configured to require a predetermined number of attempts to detect voltage at step 260 before this transition. Preferably, alarm and indicator means are coupled to various means as are known to persons of ordinary skill in the art for transmitting signals to a remote location, including but not limited to via internet, cellular telephone via short message service format (SMS) or multi-media service format (MMS), radio-frequency transmission, satellite transmission, or other similar methods.

In some embodiments, sensing system 15 then obtains readings (290), which are used to first determine whether blower 11 is on or off. Upon powering up the system, an initial sensor voltage reading is obtained followed by at least one additional reading within about five seconds. Readings consistent with a step increase to a minimum of 2-3 volts, by way of example, for the initial reading, followed by a ramping decrease to a reading that is approximately equal to the value stored in memory from the prior monitoring event, indicate that the blower is on. Conversely, readings consistent with a step increase, as above, but followed by a ramping decrease to below 1.0 volts, again by way of example, within about five seconds, indicate that sensing system 15 is operating properly, but that blower 11 is off. In such event, the system automatically ceases monitoring, and returns to standby mode until the next scheduled monitoring.

Preferably, the system is programmed to monitor on an accelerated schedule once it determines that the blower is off, rather than waiting until the next monitoring opportunity according to a normal schedule. An accelerated schedule is repeated at different and more frequent intervals than the normal schedule, and continues until an indication is received that blower 11 is on, or until a predetermined maximum number of times to run the accelerated schedule has occurred.

If the initial sensor voltage reading and the one immediately following indicate that the blower is on, then optionally, a delay is provided between steps 260 and 290 before additional readings are obtained. As previously described, when readings are obtained (290), sensing system 15 calculates an average of a plurality of readings in order to obtain a value. For example, the number of readings may be sixty-four in about a ten second period, or some other number of readings within a given period of time. At step 300, sensing system 15 determines whether the predetermined number of readings has been obtained. Minimum and maximum readings are determined, as well, and the timing of each one stored at step 305. With each reading, a reading counter is incremented until the appropriate number of readings has been obtained, at which point the reading counter is cleared at step 310.

Sensing system 15 then compares readings obtained during the particular monitoring event to determine whether blower 11 was operating at a stable speed when the readings were obtained, or whether it was in the process of starting or stopping (320). For example, if the maximum and minimum readings at step 305 differ by more than a predetermined set amount then it indicates that blower 11 was not operating at a stable speed during the entire period (e.g., ten seconds) when those readings were obtained. This may be due to the fact that blower 11 was in the process of starting up, or shutting down. In both cases, the speed of blower 11 typically will change within a relatively short period of a few seconds. In any case, such a value is discarded (325), and present monitoring ends (330) pending the next monitoring event according to an accelerated schedule (325).

In some embodiments, if sensing system 15 is configured to maintain a 5° C. temperature difference between heated sensor 30 and unheated sensor 32, then a difference of about 0.250 volts or greater between the minimum and maximum readings would cause the system to discard the sample (325). Conversely, if that difference is less than or equal to about 0.250 volts, then sensing system 15 considers the readings to be valid, and the sample and its value are not discarded (340). If a blower starts up at the instant a monitoring event begins, or while a monitoring event is ongoing, then the blower will be running fastest when the last readings are obtained. Conversely, if it shuts down at the instant a monitoring event begins, or while a monitoring event is ongoing, it will be at its slowest speed when the last readings are obtained. Accordingly, at least one reading from a group of the last ten readings (if set to obtain a value based on an average of sixty-four readings) must be reflected in the difference between maximum and minimum.

Optionally, at this point sensing system 15 compares (not shown in FIG. 5) the value obtained during a monitoring event to the value for zero air flow obtained at calibration and stored in memory. If the averages are substantially identical, for example within, about ±0.50 volts, this indicates that blower 11 is off. This prompts an accelerated schedule to be initiated, if not already started due to readings at step 320.

If it is determined that blower 11 was on when the readings were obtained, and not in the process of starting up or shutting down (320), and that a sufficient number of readings were obtained (340), then a sequence generally provided at steps 350-390 occurs next. From prior monitoring events, at least one known valid value is stored in memory, indicating voltage needed to maintain the predetermined temperature difference at a particular blower speed (360) for a particular filter condition. If many monitoring events occurred consecutively without a change in blower operating speed, then an average of the most recent of such values (up to a predetermined number) is stored.

Changes in fan speed typically occur when a HVAC system transitions from winter heating to summer cooling, or vice versa, or when the HVAC system controller changes blower speed, as programmed, to increase HVAC system efficiency. Such transitions occur frequently at various times of the year in various places in the country. One advantage of the present embodiments is that, for HVAC systems with two-speed blowers, or multi-speed blowers, a dynamic clog threshold can be provided based upon a fan speed offset value. At step 380, if a difference exists to a predetermined level, between values stored in memory at calibration representing a clean filter, on one hand, and a stored value obtained during recent monitoring events, on the other, then this is an indication that blower fan speed has changed. Depending on the positioning of sensing system 15 relative to filter 14, this difference will either represent a rise in measured voltage, or a drop in measured voltage.

For illustrative purposes, suppose the voltage measured at the time of calibration (for a clean filter) was 2.0 volts, and a clog detection threshold of 0.150 volts above the clean filter value, i.e., 2.150 volts. Further suppose, at a certain point in time, that 2.1 volts is a stored value representing an average of values from the most recent valid monitoring events (referred to as S, below). If the next monitoring event obtained a value of 2.38 volts (represented for discussion purposes as X, which is 0.280 volts greater than the S value), that would be a significant difference over both the clean filter value at calibration and the average of the most recent valid monitoring events (S).

However, sensing system 15 is configured in some embodiments to proceed to a next monitoring event, without triggering alarm mode, and without recalculating the clog threshold, and without factoring the 2.38 volts into the average of values for the most recent valid monitoring events. Instead, the system proceeds to the next monitoring event. Now, suppose that value is 2.4 volts (Y), which is again a significant upward movement. If the sensing system is configured to evaluate for fan speed change after three consecutive non-zero air flow values representing dynamic shifts in the value, then the system still would not take any action in terms of these values. Instead, the system would proceed to the next monitoring event.

Then, in the present example, only if the next monitoring event resulted in a third consecutive non-zero air flow value (Z), would the dynamic features of the system come into play. Now, suppose that the next monitoring event obtained a value of 2.42 volts. This would be the third consecutive non-zero air flow value representing an increase of at least 0.250 volts above the S value (380, 385). In that event, four things would happen. First, the sensing system would sense this as a fan speed change. Second it would discard the values that produced the 2.1 volts (S) value, which had been stored in memory. Third, it would recalculate the clog threshold, to account for the change in fan speed (390). Fourth, it would re-seed (i.e., replace in memory) the values that had previously produced the 2.1 volts (S) value (also 390).

Step 390 calculates the average value by which values X, Y, and Z exceed the S value for the current filter state. This would produce an average of (0.38 volts+0.4 volts+0.42 volts)/3=0.4 volts. The sensing system then uses this average for both calculations at step 390. Specifically, the clog threshold would increase from 2.150 volts to 2.550 volts, and the average of values from the most recent monitoring events would increase from 2.1 volts to 2.5 volts. In this way, it will be appreciated that before the fan speed change, the average of values from recent monitoring events was 0.050 volts from the clog threshold (2.150 volts-2.1 volts), and after the fan speed change it is the same (2.550 volts-2.5 volts).

At step 400, a value is obtained for the sample and compared to a clog threshold stored in memory. For example, this threshold may be set at 0.150 volts above or below a stored value of a clean filter based on calibration readings. If it is determined at step 400 that the average of the readings obtained is greater (or less, depending on the upstream/downstream position of the sensing system) than this clog threshold, then a clog counter is incremented at step 420. At step 430, if the clog counter has consecutively exceeded the clog threshold a predetermined number of times, for example three times, then system 15 activates an alarm or a similar signal at step 440, which may include one or more visual or audible indications. If, however, it is determined at step 400 that the average is less than or equal to the clog threshold, or if the clog counter has not exceeded the maximum number of times that is set (430), then sensing system 15 returns to its normal schedule as indicated by step 405. In such case, power to the sensing system is discontinued to at step 410 (until the next scheduled monitoring event) and transition is made to standby mode (440).

By programming processor 29 to compare readings and values, and discard certain values while using others, as described herein, it removes the need to have separate sensors detecting either the speed of the motor attached to blower (11) or of the air flow in duct 10, or both. Further, it enables sensing system 15 to determine the condition of filter 14 without requiring substantially high temperature increases between sensors 30 and 32, respectively.

As shown in FIG. 6, a graph plots sensor voltage level against the level of cleanliness (i.e., in terms of the absence of dirtiness and/or clogging) of a gas filter in a HVAC system. Moving from along various points shown on the graph provides a continuum from a clean filter at the time of replacement or cleaning (A), to a dirty filter (B), to a moderately clogged filter (C), to a substantially clogged filter (D), and finally to a completely blocked filter (E) in which no air passes through. Preferably, a clog threshold is determined between points B and C. In some embodiments, that threshold is responsive to the stable operating speed of a blower at the time when voltage readings are obtained.

It will be understood that the embodiments described herein are not limited in their application to the details of the teachings and descriptions set forth, or as illustrated in the accompanying figures. It will be understood by those having ordinary skill in the art that modifications and variations of these embodiments are reasonably possible in light of the above teachings and descriptions. The descriptions herein are not intended to be exhaustive, nor are they meant to limit the understanding of the embodiments to the precise forms disclosed. Accordingly, the foregoing descriptions of several embodiments and alternatives are meant to illustrate, rather than to serve as limits on the scope of what has been disclosed herein.

Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use herein of “including,” “comprising,” “e.g.,” “containing,” or “having” and variations of those words is meant to encompass the items listed thereafter, and equivalents of those, as well as additional items. 

We claim:
 1. A filter clog detection system for a gas filter in a gas conduit operatively attached to a gas blower having power to produce a gas flow, comprising: a sensing system that measures a property within the conduit based upon a flow of gas into the sensing system from outside the gas conduit, wherein a condition of the gas filter affects the value of the measured property; a controller, including a memory that stores electronic information therein and a processor executing program instructions, that obtains a first measurement of the property when the gas filter is in a first state, obtains a second measurement of the property when the gas filter is in a second state, and compares the first measurement and the second measurement; wherein the sensing system comprises a sensing tube formed with an opening at two ends, and a plate attached to the sensing tube, the sensing tube having an inlet as one of the openings that receives gas flow from outside the gas conduit, and an outlet as the other opening that allows gas to flow into the gas conduit; wherein the sensing system further comprises a first sensor linked with a heater that provides energy to the first sensor; a second sensor that senses a first temperature within the sensing tube, wherein the first sensor and second sensor are positioned inside the sensing tube and exposed to gas flowing through the sensing tube, wherein the heater increases voltage to the first sensor as air velocity moving through the sensing tube increases; wherein the first temperature is the temperature of gas in the sensing tube, and the measured property is the voltage for raising the temperature of the first sensor to a second temperature that is a fixed number of degrees above the first temperature; and processor means for comparing the difference between the first measurement and the second measurement to a stored value in order to validate the second measurement.
 2. The system of claim 1, wherein the second temperature is raised by no more than about 5° C. above the first temperature.
 3. The system of claim 1, wherein the controller discards the second measurement if the blower is off or is not operating at a stable operating speed.
 4. The system of claim 1, wherein the controller determines if a change in fan speed of the gas blower at the time of the second monitoring event compared to a previous monitoring event exceeds a fan speed change threshold.
 5. The system of claim 1, wherein the processor compares the difference between the first measurement and the second measurement to a clog threshold.
 6. The system of claim 5, wherein the processor recalculates the clog threshold in response to changes in fan speed of the gas blower.
 7. A clogging detection method for a gas filter in a conduit operatively attached to a gas blower having power to produce a gas flow comprising: providing a sensing tube having a first sensor coupled to a heater and a second sensor that senses a first temperature within the sensing tube when gas is flowing from outside the conduit into the sensing tube, wherein the sensing tube introduces gas flow into the gas conduit with an inlet that receives gas flow from outside the gas conduit, and an outlet that allows gas to flow into the gas conduit; providing energy from the heater to the first sensor to raise the temperature to a second temperature, which is a fixed number of degrees above the temperature measured by the second sensor, and wherein as air velocity moving through the sensing tube increases the heater increases voltage to the first sensor to raise the temperature of the first sensor; obtaining a first measurement to determine a property within the conduit when the gas filter is in a first state, wherein a condition of the gas filter affects the value of the measured property; obtaining a second measurement of the property when the gas filter is in a second state, wherein the first measurement an second measurement are voltage for raising the temperature of the first sensor to a second temperature that is a fixed number of degrees above the first temperature; and configuring a controller, having a memory that stores electronic information therein and a processor executing program instructions, to store and compare the first measurement and the second measurement, and to validate the second measurement by comparing the second measurement to a previous monitoring event to verify the blower is on and is operating at a stable operating speed.
 8. The method of claim 7, wherein the temperature of the first sensor is raised by no more than about 5° C. above the first temperature.
 9. The method of claim 7, wherein the controller recalculates the threshold in response to changes in fan speed of the gas blower.
 10. The method of claim 7, further comprising determining if a change in fan speed of the gas blower compared to a previous monitoring event exceeds a fin speed change threshold. 